1. Field of the invention
The present invention relates to a hydrogen absorbing alloy, and more specifically, relates to a hydrogen absorbing alloy suitable as a negative electrode of an alkaline storage battery.
2. Description of the related art
Since a hydrogen absorbing alloy which charges and discharges hydrogen was discovered, the application thereof has been developed not only as hydrogen storage means but also for heat pumps and batteries. Furthermore, with the development of the electronic technology, down-sizing and labor saving of electronics have become an important factor. In addition, storage batteries using the hydrogen absorbing alloy have been recently used in highlands and cold areas, and batteries having an excellent low temperature property have been desired.
In particular, an alkaline storage battery which uses the hydrogen absorbing alloy as a negative electrode has been made practicable, and the hydrogen absorbing alloy used for it has been improved one after another. Although the LaNi.sub.5 alloy (reference to Japanese Provisional Patent Publication No. 51-13934) has an advantage that the amount of hydrogen absorbed is large, it is easily cracked due to repetition of charge and discharge of hydrogen. Hence, it has such a disadvantage that the specific surface area of the alloy increases. As a deteriorated area increases, it is easily corroded by the alkaline solution.
The disadvantage has been improved by substituting a part of La of the LaNi.sub.5 alloy with rare-earth metal elements such as Ce, Pr, Nd or the like, and/or substituting a part of Ni of the LaNi.sub.5 alloy by metal such as Co, Al, Mn or the like (for example, Japanese Provisional Patent Publication Nos. 53-48918, 54-64014, 60-250558, 61-233968, and 62-43064).
And, as an alloy in which a part of La is substituted by Ce or the like, there are used a misch metal (Mm) containing 40 to 50 wt % (`wt %` is short for `% by weight`) Ce and a La-rich misch metal (Lm) containing 50 to 60 wt % La and 10 to 30 wt % Ce.
In general, when a LaNi.sub.5 type hydrogen absorbing alloy is used for batteries, it is essential that it has a high capacity, an excellent discharge property at low temperatures, and a long cycle life for repetition of charge and discharge.
In the above view, it was tried to attain the object by substituting a part of La by Ce. The cycle life for repetition of charge and discharge has been improved. However, the high rate discharge property, which is performed at a large current discharge, and discharge property at low temperatures have not been improved. Furthermore, it is found that by substituting Ni by Co, Mn, Al or the like, the high rate discharge property and the cycle life at an ordinary temperature are improved.